An association between low serum potassium concentrations and ventricular arrhythmias has been observed by a number of investigators in patients with both acute myocardial infarction and symptomatic angina. The increased frequency of ventricular fibrillation in patients with low serum potassium concentrations has not been attributable to differences in the size or location of the infarct or concomitant digitalis administration. Although prior diuretic usage is frequently associated with a low serum potassium concentration, diuretic usage in the absence of significant changes in serum electrolytes is not associated with an increase in malignant ventricular arrhythmias. It is not clear whether the alterations in serum potassium are important in the pathogenesis of the arrhythmias or merely a marker for other arrhythmogenic factors. To the extent that changes in serum potassium contribute to the production of serious ventricular arrhythmias, efforts should be directed at the correction of such electrolyte disturbances. Attempts to increase the serum potassium in acute cases do not appear to be successful in preventing ventricular arrhythmias in this setting. Primary prevention of electrolyte disturbances would seem to offer more effective protection against serious arrhythmias. Long term studies with beta-blockers support this premise.